The difference is huge.
First , the massionete foundation has to be reinforced. Lazima the house to sit on chuma. Pray your plot is not cotton coz you have to excavate all the cotton or suspend the massionte, and then back fill. The bangalow has less weight , so with a foundation made of 9 inch stones you are good to go. Therefore Significant savings for the bangalow.
Although the sub structure of the bangalow occupies a bigger area, the massionte has a super structure as well. Again significant bangalow savings .
The massionte is has columns reinforced with round bars as per engineer design . The bangalow does not. Again bangalow saves costs significantly.
The superstructure whether wholly concrete or horrow blocks, the amount of steel required is Phenemonal . The ballast is expensive and labour is significant.bangalow saves huge again .
The bangalow roof area is bigger, but the masionete roof is usually over designed to complement the compressed built area and the corners that masionete mostly have, this means higher labour, more timber . Although masionete wins this one, its important to note ,the savings are not significant.
Less area occupied by massionte . Advantage but not a saving.
Overall , a bangalow of similar built area with a massionete , should be able to give the developer at least 50 % savings when compared to similar area but massionete.
Out of experience.
Constructing a 3br bangalow on msa road or Thika road, fully completed, good finishes ie ceramic tiles, all ensuite, coloured corrugated sheets not tiles , = approx 2.5 m.
Massionete , 4 brm, similar area with above, same quality interior finish and same roofing material, = approx 4.5M.
Both costs excludes land .note , I have included a 4 brm massionete . Rarely a massionete will be 3brm unless a semi detached unit.
My 2 cents.